For many years conventional display devices have been designed to be viewed by multiple users simultaneously. The display properties of the display device are made such that viewers can see the same good image quality from different angles with respect to the display. This is effective in applications where many users require the same information from the display—such as, for example, displays of departure information at airports and railway stations. However, there are many applications where it would be desirable for individual users to be able to see different information from the same display. For example, in a motor car the driver may wish to view satellite navigation data while a passenger may wish to view a film. These conflicting needs could be satisfied by providing two separate display devices, but this would take up extra space and would increase the cost. Furthermore, if two separate displays were used in this example it would be possible for the driver to see the passenger's display if the driver moved his or her head, which would be distracting for the driver. As a further example, each player in a computer game for two or more players may wish to view the game from his or her own perspective. This is currently done by each player viewing the game on a separate display screen so that each player sees their own unique perspective on individual screens. However, providing a separate display screen for each player takes up a lot of space and is costly, and is not practical for portable games.
To solve these problems, multiple-view directional displays have been developed. One application of a multiple-view directional display is as a ‘dual-view display’, which can simultaneously display two or more different images, with each image being visible only in a specific direction—so an observer viewing the display device from one direction will see one image whereas an observer viewing the display device from another, different direction will see a different image. A display that can show different images to two or more users provides a considerable saving in space and cost compared with use of two or more separate displays. FIG. 1 illustrates a dual view display 1 installed in a motor vehicle. The display is displaying one image (GPS satellite navigation data) to the driver and at the same time is displaying a second image (a film) to a front seat passenger. The driver cannot see the film displayed to the passenger, and the passenger cannot see the satellite navigation data displayed to the driver.
Examples of possible applications of multiple-view directional display devices have been given above, but there are many other applications. For example, they may be used in aeroplanes where each passenger is provided with their own individual in-flight entertainment programmes. Currently each passenger is provided with an individual display device, typically in the back of the seat in the row in front. Using a multiple view directional display could provide considerable savings in cost, space and weight since it would be possible for one display to serve two or more passengers while still allowing each passenger to select their own choice of film.
A further advantage of a multiple-view directional display is the ability to preclude the users from seeing each other's views. This is desirable in applications requiring security such as banking or sales transactions, for example using an automatic teller machine (ATM), as well as in the above example of computer games.
A further application of a multiple view directional display is in producing a three-dimensional display. In normal vision, the two eyes of a human perceive views of the world from different perspectives, owing to their different location within the head. These two perspectives are then used by the brain to assess the distance to the various objects in a scene. In order to build a display which will effectively display a three dimensional image, it is necessary to re-create this situation and supply a so-called “stereoscopic pair” of images, one image to each eye of the observer.
Three dimensional displays are classified into two types depending on the method used to supply the different views to the eyes. A stereoscopic display typically displays both images of a stereoscopic image pair over a wide viewing area. Each of the views is encoded, for instance by colour, polarisation state, or time of display. The user is required to wear a filter system of glasses that separate the views and let each eye see only the view that is intended for it.
An autostereoscopic display displays a right-eye view and a left-eye view in different directions, so that each view is visible only from respective defined regions of space. The region of space in which an image is visible across the whole of the display active area is termed a “viewing window”. If the observer is situated such that their left eye is in the viewing window for the left eye view of a stereoscopic pair and their right eye is in the viewing window for the right-eye image of the pair, then a correct view will be seen by each eye of the observer and a three-dimensional image will be perceived. An autostereoscopic display requires no viewing aids to be worn by the observer.
An autostereoscopic display is similar in principle to a dual-view display. However, the two images displayed on an autostereoscopic display are the left-eye and right-eye images of a stereoscopic image pair, and so are not independent from one another. Furthermore, the two images are displayed so as to be visible to a single observer, with one image being visible to each eye of the observer.
In a multiple view directional display, the formation of the viewing windows is typically due to a combination of the picture element (or “pixel”) structure of the image display unit of the display and an optical element, generically termed a parallax optic. An example of a parallax optic is a parallax barrier, which is a screen with transmissive regions, often in the form of slits, separated by opaque regions. This screen can be set in front of or behind a spatial light modulator (SLM) having a two-dimensional array of picture elements to produce a multiple view directional display.
FIG. 2 is a plan view of a conventional multiple view directional device, in this case a dual view display, described in UK patent application GB 2 405 542. The directional display 1 consists of a spatial light modulator (SLM) 2 that constitutes an image display device, and a parallax barrier 3. The SLM 2 of FIG. 2 is in the form of a liquid crystal display (LCD) device having an active matrix thin film transistor (TFT) substrate 4, a counter-substrate 6, and a liquid crystal layer 5 disposed between the substrate 4 and the counter substrate 6. The SLM is provided with addressing electrodes (not shown) which define a plurality of independently-addressable picture elements (or pixels), and is also provided with alignment layers (not shown) for aligning the liquid crystal layer. The pixels of the SLM 4 are arranged in rows and columns with the columns extending into the plane of the paper in FIG. 2. Other components such as viewing angle enhancement films and linear polarisers may be provided, and these have been omitted from FIG. 2 for clarity. Illumination is supplied from a backlight (not shown), which provides substantially uniform illumination over the area of the SLM 4.
The parallax barrier 3 comprises a substrate 7 with a parallax barrier aperture array formed on its surface adjacent the SLM 2. The aperture array comprises vertically extending (that is, extending into the plane of the paper in FIG. 1) transparent apertures 8 separated by opaque portions 9. The parallax barrier may comprise additional components, such as, for example, an anti-reflection (AR) coating on the output surface of the parallax barrier substrate 7 (which forms the output surface of the display 1).
In use, two images are displayed by the SLM, interlaced between alternate columns of pixels. The display 1 forms a left image and a right image, and these are directed to a left observer 10 and a right observer 10′ respectively.
A parallax barrier is the most common type of parallax optic used in current dual view and autostereoscopic 3-D displays. However, use of a parallax barrier has a number of disadvantages. Firstly, as shown in FIG. 2, a parallax barrier comprises opaque portions (which are usually absorbing for visible light) alternating with transmissive portions, and the effect of the opaque portions is to reduce the overall brightness of the display. The viewing windows obtained by use of a parallax barrier are relatively small, so that an observer has relatively little freedom in where they position their head—and a small movement of the observer's head is likely to take the head outside the viewing window. A further disadvantage is that an observer who views the display along the axis perpendicular to the display face of the display is likely to see both displayed images.
Okumura, Tagaya and Koike describe, in “Highly efficient backlight for liquid crystal display having no optical films”, Applied Physics Letters, Vol. 83, No. 13, p 2515, 2003, the current state of the art in backlight construction. They describe a backlight having a waveguide with an upper surface which is roughened so that light is emitted over the entire area of the upper surface of the waveguide. Light-directing films are provided between the waveguide of the backlight and an image display device, to direct light emitted from the waveguide substantially along the axis of the image display device.
JP-A-8 110 495 discloses a multiple view display having a parallax barrier made from reflective metal. Light that is blocked by the opaque regions of the parallax barrier is reflected back towards the backlight and is ultimately and is “recycled” (i.e., is reflected towards the display again) by the backlight. The efficiency of this technique is however expected to be low, because, every time light is reflected and recycled, the metal regions of the parallax barrier absorb a significant amount of light. Light is also absorbed in components of the backlight, such as diffusers.
FIG. 18 shows a prior art 3-D display, described in U.S. Pat. No. 5,854,706, which does not use a parallax barrier. First and second images 11,12 are displayed on an image display layer. A single half silvered mirror 16 is positioned at the centre of the image display layer, and extends perpendicularly away from the image display layer. A first linear polariser 13 is placed over the region of the image display layer displaying the first image 11, and a second linear polariser 14, having its transmission axis orthogonal to the transmission axis of the first polariser, is placed over the region of the image display layer displaying the second image 12. Louvres 15,19, inclined with respect to the image display layer, are positioned over the image display layer. An observer 18 wears polarising glasses 17 such that they see only one of the images in each eye and so perceive a 3-D image.
The display of FIG. 18 requires that the observer wears polarising glasses to obtain a 3-D effect.